Add `volunteer/leaderboard` endpoint

[TimJentzsch]

Add a volunteer/leaderboard endpoint to get the data necessary for the /leaderboard command (GrafeasGroup/buttercup#18).

For reference, the bot output will look something like this:

The endpoint could have a format like this:

{
  "top": [ ... ],
  "above": [ ... ],
  "user": { ... },
  "below": [ ... ]
}

• The top field contains a list of users at the top of the leaderboard. The amount of users shown there can be controlled by the top query parameter.
• The user field contains the user that was queried for via the user parameter.
• The above field contains a list of users that are higher on the leaderboard than the user. The amount can be controlled by the above query parameter.
• The below field contains a list of users that are lower on the leaderboard than the user. The amount can be controlled by the below query parameter.

Each user entry should have a form similar to this:

{
  "id": 4034,
  "username": "Tim3303",
  "rank": 16,
  "gamma": 1248
}